Pyschomanteum, is the Mirror casting form of Divination. Most commonly people use this form to ask those in the spiritual realm for information or to contact them in general. Normally when a Diviner would view into pool or anything containing water ((preferably, a pool, lake, stream, basin containing water,etc))They'd then recieve visions, or see spritual entities .
This would be achieved due to the fact the Diviner would have already studied Water scrying (Hydromancy) and would know how to decode any type of images seen through this still pool of water.
I found an interesting article this morning that goes a little further: My Experience in the Psychomanteum